Many of today's computers, cellular telephones, and other such devices rely on wireless communication to exchange information with other devices, such as to surf the Internet, send and receive email, and communicate audio and video. A modern cellular telephone, for example, may include a cellular phone modem to place and receive calls, a GPS receiver to determine physical location, a Wi-Fi interface that enables the device to join a Wi-Fi network and surf the Internet, as well as a Bluetooth interface that couples to an earpiece or car audio system for making phone calls.
Cellular telephones and other such devices have in only a couple decades gone from being obscure and expensive luxuries to replacing traditional land-line telephones. It is increasingly common for people to carry a cellular phone with them nearly wherever they go, constantly having access to not only phone functionality, but also the ability to send text messages and emails, use GPS or mapping features, and run a variety of applications.
GPS is useful in that it enables a person to determine where they are, overlay their location on a map, and get directions. GPS signals are often also used by other applications, such as a restaurant review application that shows reviews of restaurants near a user, or a shopping application that shows stores near a user that have a particular item for sale.
But, GPS signals are difficult to receive in some cities with tall buildings or other interference, and GPS is often left out of lower cost devices. Some devices therefore use cellular phone towers or other radio signals having known location to use as a reference, and can determine approximate position of a mobile radio device by estimating the distance from the mobile wireless device to multiple cellular towers. But, factors such as multipath signals where a cellular phone tower's signal is reflected off a building or other object can complicate methods such as these, resulting in inaccurate distance determinations and position determination.
For these and other reasons, it is desired to accurately measure the RF signal propagation delay between two devices.